Design: The main, clinical, experiment has a parallel, repeated-measures design, where four groups (two each of healthy subjects and participants with TBI) will encode pictures, words and rules and then exercise at either a high or very low (placebo) intensity. Recall will be tested one hour and again at seven days after exercise. Blood and saliva will be collected before and immediately after exercise and assayed for biomarkers of exercise, thought to be possible mediators of the memory effect. In a parallel-design fMRI experiment, intended to explore the brain basis of the effect of exercise on memory, healthy volunteers will view pictures, exercise at a high or low intensity, and then perform a recall task in the scanner. The analysis will look for differences in location and extent of evoked brain activations evoked by picture recall after low and high intensity exercise.